In the most common positive displacement hydraulic machines the fluid chambers undergo cyclical variations in volume following a roughly sinusoidal function. It is known from EP0361927 that a chamber can be left to idle by holding an electromagnetically actuated valve, between the working chamber and the low-pressure source, in the open condition. Thus the output of such synthetically commutated machines is varied through the action of first filling each working chamber with liquid, then deciding whether to reject the liquid back to the low-pressure source or to pump it at pressure to the output manifold. Pumping the liquid back to the low-pressure source means that a very small amount of power needs to be expended, during the time that a working chamber is idle, whilst still allowing the working chambers to become productive with a minimum latency period. EP0494236 introduced an additional operating mode which allows the use of the hydraulic machine in a motoring cycle where torque is applied to the rotating shaft, thus allowing a controllable bi-directional energy flow.
WO/2008/012587 introduced a method to adjust the working chamber cycles of such machines so as to react very rapidly to events by adding or subtracting torque (or flow) from the machine's shaft (or outlets). The adjustment causes additional or lesser torque (or flow) compared to the original and underlying demanded torque (or flow).
The designer wishing to use machines of the aforementioned kind (for example due to their unsurpassed efficiency and controllability) faces new challenges which are not found when using machines in which every working chamber is active each revolution and to largely the same degree as each other. These challenges include differential wear, shaft load balancing, and resonance. All of these challenges are caused by the sometimes irregular displacement of fluid and application of torque to the rotating shaft and are described in more detail in the following paragraphs.
Differential wear: over time, synthetically commutated machines may use some cylinders statistically more often than others. This is particularly a problem when certain cylinders are connected at times to different working functions, for example as described in WO/2008/009950. This leads to a reduction in the lifetime of the machine, as favoured working chambers may fatigue or wear faster than infrequently used ones.
Shaft load balancing: many conventional fluid working machines are designed so that radial forces applied by the many simultaneously active working chambers to the rotating shaft are balanced or offset against each other. However, where some working chambers are disabled as in synthetically commutated machines, it is possible for very high unbalanced forces to increase wear or even damage the bearings holding the shaft, or even to bend the shaft itself.
Resonance: applications where the irregular application of torque to a shaft is undesirable include transmissions such as vehicle and wind turbine transmissions, while applications where irregular displacement of fluid is undesirable include those with working functions such as mobile and fixed hydraulically driven materials handling equipment (forklifts, excavators and the like). In these applications, irregularity can excite or exacerbate mechanical resonances inherent in the structures and parts driven by or driving the fluid working machine. For example, irregular drive of a wheel might induce undue torsional vibration at the natural frequency of an axle, or the irregular force from a so-driven wheel reacting against the ground may induce undue structural vibration at the natural frequency of parts of a vehicle chassis. In material handling equipment the irregularity of displacement of fluid into a hydraulic actuator might induce undue vibration at the natural frequency of a forklift's tower or the extendable arm of an excavator, for example.
From the above it will be appreciated that there are numerous applications in which careful control of the output torque or fluid displacement from a synthetically commutated fluid working machine is very important. The present invention aims to address various of the problems described above.